Hydrodynamic bearing, method of manufacturing the same, method of manufacturing shaft member for hydrodynamic bearing, spindle motor, and recording disk driving apparatus

ABSTRACT

According to the invention, a shaft as a component of a shaft member of a hydrodynamic bearing and a disc member such as a thrust plate are joined to each other by welding by a simple method with high precision. A shaft having a cylindrical outer circumferential surface, in joint surfaces which are joined to each other of the shaft and a disc member having a flat surface facing an end surface of the shaft, a circumferential projection having a diameter smaller than the outside diameter of the shaft and projected in the axial direction and a recess at least of which outer periphery has a diameter smaller than the outside diameter of the shaft and larger than the diameter of the projection and has a circular shape are provided. By applying a predetermined voltage across the shaft and the disc member in a state where the joint surface of the shaft and the joint surface of the disc member are in contact with each other, the projection is melted. The melted matter is housed in the recess. At the time of the welding, the end surface of the shaft and the flat surface of the disc member are in contact with each other in a portion outside of the recess, and perpendicularity of the disc member to the shaft is set with high precision. At the time of joining the shaft and the disc member, the thrust plate is held by an axis adjusting jig made of a resin to position the center position of the thrust plate, thereby eliminating an influence of a process tolerance or the like and obtaining excellent coaxiality with the shaft.

CROSS REFRENCE TO RELATED APPLICATION

[0001] This application is based on and incorporates herein by referenceJapanese Patent Applications No. 2003-116714 filed on Apr. 22, 2003 andNo. 2004-055771 filed on Mar. 1, 2004.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a hydrodynamic bearing in whicha shaft member is constructed by joining and fixing a disc member suchas a disc-shaped thrust plate to a shaft and a method of manufacturingthe same, an apparatus of manufacturing a shaft member for ahydrodynamic bearing, a spindle motor having the hydrodynamic bearing,and a recording disk driving apparatus for rotating a recording disk bya spindle motor.

[0004] 2. Background Information

[0005] Hitherto, as a bearing of a motor for rotating a recording diskin a hard disk driving apparatus, a removable disk driving apparatus, orthe like, various hydrodynamic bearings for rotatably supporting a rotorby generating a dynamic pressure by a lubrication fluid such as oil heldin a space between a shaft and a sleeve at the time of rotation of themotor have been proposed. Such a hydrodynamic bearing is constructed by,for example, a radial hydrodynamic bearing part and a thrusthydrodynamic bearing part. A shaft part (shaft member) is constructed bya shaft having a cylindrical outer circumferential surface and a thrustplate (disc member) provided so as to be substantially orthogonal to theaxis of the shaft. The radial hydrodynamic bearing part is constructedin the outer circumferential surface of the shaft, and the thrusthydrodynamic bearing part is constructed in the flat surface of thethrust plate.

[0006] As a method of fixing the shaft and the thrust plate to eachother, for example, a method disclosed in Japanese Unexamined PatentPublication (JP-A) No. 2000-324753 is known. In the method, the thrustplate is formed in an annular shape and is press fit to the outercircumferential surface at one end of the shaft, and a joint partbetween the end surface of the shaft and the thrust plate is fixed bylaser welding. There is another known method disclosed in JP-A No.2003-097545 in which a screw part is provided for each of the outercircumferential surface of an end portion of a shaft and the innercircumferential surface of an annular thrust plate and the screw partsare screwed to each other, thereby fixing the shaft and the thrust plateto each other.

[0007] In recent years, application of a recording disk drivingapparatus used for an apparatus such as a personal computer to a smallerportable information terminal has been started. A spindle motor mountedon a driving apparatus of this kind is requested to have high-speed andhigh-precision rotation which are conventionally demanded and, inaddition, smaller size, reduced thickness, lower cost, and lower powerconsumption.

[0008] To address such requests, however, the dimension in the axialdirection of the shaft has to be shortened. In the case of using bothpress fitting and laser welding disclosed in JP-A No. 2000-324753 andthe case of fixing the shaft and the thrust plate by screwing disclosedin JP-A No. 2003-097545, to maintain the perpendicularity of the thrustplate to the axis of the shaft with high precision, the thrust plate hasto be thick to a certain extent. Consequently, it is difficult tosufficiently assure the support length of the shaft by the radialhydrodynamic bearing part.

[0009] In a spindle motor, holding of the posture of the rotor such aswhirling during rotation of the rotor on which a recording disk ismounted thoroughly depends on the radial hydrodynamic bearing part.Therefore, to stably hold the posture of the rotor, the support lengthof the shaft by the radial hydrodynamic bearing part has to besufficiently assured. It is, however, very difficult to make the motorsmaller and thinner as a whole while maintaining the requested rotationprecision.

[0010] JP-A No. 2002-168240 discloses a method of integrally forming ashaft and a thrust plate in order to reduce the thickness of the thrustplate. JP-A No. 2003-056567 discloses a method of fixing a shaft and adisc-shaped thrust plate by performing resistance welding on the axialpart.

[0011] According to the method of integrally forming the shaft and thethrust plate (JP-A No. 2002-168240) and the method of fixing the shaftand the disc-shaped thrust plate by resistance welding (JP-A No.2003-056567), the motor can be made smaller and thinner as a whole whilemaintaining the requested rotation precision. However, there are stillthe following technical problems.

[0012] Consequently, a molding method by casting is not suitable for amember as a component of the hydrodynamic bearing, such as the shaft andthe thrust plate. The reason is that a number of small holes are formedin the surface of the member by casting and, in the case of performing afinishing process by cutting the surface of the shaft or thrust plate(surface precision process) after the casting process, metal particlessuch as powders generated at the time of cutting enter the small holesand cannot be easily removed perfectly by cleaning. If the hydrodynamicbearing is used while the metal particles such as powders remain on thesurface of the members, the metal particles in the small holes aregradually raked out by the flow of a lubrication fluid by rotation andare mixed into the lubrication fluid, so that it causes problems such asseize, damage, and the like of the bearing.

[0013] Consequently, a method of forming the shaft and the thrust plateby a cutting process of, for example, cutting a metal rod member iscommon. In the case of forming the shaft and the thrust plate integrallyas disclosed in JP-A No. 2002-168240, a metal rod member having adiameter larger than that of the thrust plate extended in a flange shapefrom the outer circumferential surface of the shaft is cut, so that longtime is required for the processing, the yield deteriorates, andproductivity decreases. Since the member is much wasted, it may disturbreduction in the cost.

[0014] In addition, in the case of integrally forming the shaft and thethrust plate, when a finishing process is performed on the flat surfaceof the thrust plate as a component of the thrust hydrodynamic bearingpart, a grinding relief for finishing the bearing surface by grindinghas to be provided for the root portion of the shaft (the corner portionbetween the shaft and the thrust plate). The grinding relief is createdby a process of preliminarily removing a portion with which a grindingstone does not come into contact, of the corner in the root portion.Consequently, to assure the area of the flat surface of the thrust plateand obtain necessary support rigidity in the thrust direction, thediameter of the thrust plate has to be also increased. Therefore, theviscous resistance of the lubricant fluid at the time of rotation in theradial hydrodynamic bearing part and the thrust hydrodynamic bearingpart increases, and the rotation load of the motor increases, so that apower consumption amount also increases.

[0015] In the method of fixing the shaft and the disc-shaped thrustplate by resistance-welding the axial portions of them (JP-A No.2003-056567), problems such as increase in the cost and powerconsumption amount like in the case of integrally forming the shaft andthe thrust plate can be avoided. However, since the resistance weldingis performed in a state where a projection provided around the axis ofthe shaft and the thrust plate are in point-contact with each other, thestress and heat occurring when the shaft and the thrust plate arepressurized and pressed against with each other are concentrated in theprojection portion of the shaft. It is therefore feared that adeformation such as deflection occurs in the case of using a thin thrustplate. Further, since the resistance welding is performed around theaxis of the shaft, it is difficult to assure a sufficient welding areaand it is also feared that the welding strength varies. If a heavyvoltage is applied or current carrying time is increased to prevent suchvariations in the welding strength, new problems occur such as meltingof an electrode contact portion between the shaft and the thrust plateand occurrence of so-called weld dusts created when the temperature inthe welded portion becomes too high and a melted metal drifts.

[0016] When deflection occurs in the thrust plate as a component of thethrust hydrodynamic bearing part, naturally, the support precisiondeteriorates, so that the thrust plate cannot rotate stably. Since theweld dusts cannot be completely removed even by cleaning like the metalparticles, it may cause seize or damage in the bearing part.

[0017] Further, in the case of joining the shaft and the thrust plate toeach other by resistance welding, to eliminate the influence ofvariations in dimensional precision due to process tolerances and tojoin the members at high precision to an extent that it does not exertan influence on the axis support, it is necessary to absorb thevariations in dimensional precision by an axis adjusting jig foradjusting the axes of the shaft and the thrust plate at the time ofwelding.

[0018] The problems as described above occur not only in the case ofjoining the thrust plate as a component of the thrust hydrodynamicbearing part to the shaft as a component of the radial hydrodynamicbearing part but also in the case of joining another disc member such asa disc-shaped member for preventing coming off to the shaft as acomponent of the radial hydrodynamic bearing part.

SUMMARY OF THE INVENTION

[0019] An object of the invention is to provide a hydrodynamic bearinghaving a shaft member constructed by fixing a disc member to an endsurface of a shaft and a bearing member constructing a radialhydrodynamic bearing part in cooperation with the shaft member andcapable of rotating relative to the shaft member, realizing smaller sizeand reduced thickness of the bearing as a whole and lower cost and lowerpower consumption while maintaining required rotation precision.

[0020] To achieve the object, according to the invention, a shaft havinga cylindrical outer circumferential surface, in joint surfaces which arejoined to each other of the shaft and a disc member having a flatsurface facing an end surface of the shaft, a circumferential projectionhaving a diameter smaller than the outside diameter of the shaft andprojected in the axial direction and a recess at least of which outerperiphery has a diameter smaller than the outside diameter of the shaftand larger than the diameter of the projection and has a circular shapeare provided. By applying a predetermined voltage across the shaft andthe disc member in a state where the joint surface of the shaft and thejoint surface of the disc member are in contact with each other, theprojection is melted. The melted matter is housed in the recess. The endsurface of the shaft and the flat surface of the disc member are incontact with each other in a portion outside of the recess, and theshaft and the disc member are integrated by welding.

[0021] The projection may be provided for the end surface of the shaftor the flat surface of the disc member. The recess may be provided inthe end surface of the shaft or the flat surface of the disc member. Atthe time of welding the shaft and the disc member, it is important tomelt the projection and house a melt matter created by the melting intothe recess. The end surface of the shaft and the flat surface of thedisc member come into contact with each other in the portion outside ofthe recess in the process of melting the projection, thereby enablingthe shaft and the disc member to be integrally fixed to each other bywelding in a state where the flat surface of the disc member isorthogonal to the axis of the shaft.

[0022] Another object of the invention is to provide a method ofmanufacturing a hydrodynamic bearing realizing smaller size, reducedthickness, and lower cost while maintaining required support rigidityand precision, and a method of manufacturing a shaft member used for thehydrodynamic bearing.

[0023] Further another object of the invention is to provide anapparatus for manufacturing a hydrodynamic bearing capable of holding ashaft and a disc member such as a thrust plate joined by resistancewelding with high concentricity and realizing high-precision joining.

[0024] Further another object of the invention is to provide a spindlemotor realizing smaller size and reduced thickness as a whole whilemaintaining required rotation precision and also realizing lower costand lower power consumption.

[0025] Further another object of the invention is to provide a recordingdisk driving apparatus on which a spindle motor realizing smaller sizeand reduced thickness as a whole while maintaining required rotationprecision and also realizing lower cost and lower power consumption ismounted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Referring now to the attached drawings which form a part of thisoriginal disclosure:

[0027]FIG. 1 is a cross section showing a schematic configuration of ahydrodynamic bearing according to the invention and a spindle motorusing the same.

[0028]FIG. 2 is a cross section schematically showing a sleeve in FIG.1.

[0029]FIG. 3 is a schematic diagram showing a spiral groove formed in athrust hydrodynamic bearing part in FIG. 1.

[0030]FIG. 4 is a schematic configuration diagram of an apparatus usedfor joining a shaft and a thrust plate.

[0031]FIG. 5 is an enlarged view of a portion around joint surfaces ofthe thrust plate and the shaft.

[0032]FIG. 6 is an enlarged view of a portion around the joint surfacesof the thrust plate and the shaft as a modification of the embodiment ofFIG. 5.

[0033]FIGS. 7A and 7B are enlarged views of a portion around the jointsurfaces of the thrust plate and the shaft as another modification ofthe embodiment of FIG. 5, and show a state just before joint and a stateafter completion of welding, respectively.

[0034]FIG. 8 is a cross section schematically showing the internalconfiguration of a disk driving apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Embodiments of the invention will be described with reference tothe drawings.

[0036] (1) Configuration of Spindle Motor

[0037] A spindle motor according to an embodiment of the invention shownin FIG. 1 has: a rotor including a rotor hub 2 in which a hard disk(indicated as a disk plate 53 in FIG. 8) is held in a peripheralportion, a shaft 4 having a cylindrical outer circumferential surfaceattached to the rotor hub 2, and a disc-shaped thrust plate 6 as a discmember extended from an outer circumferential surface of a free end (anend on the side opposite to the side attached to the rotor hub 2) of theshaft 4 to the outside in the radial direction; and a sleeve 10 fixed toa cylindrical boss 8 a provided for a bracket 8. A rotor magnet 12 isattached to the inner surface side of the rotor hub 2 by means such asbonding, and a stator 14 is disposed for the bracket 8 so as to face therotor magnet 12 in the radial direction.

[0038] In the sleeve 10, as shown in FIG. 2, a through hole 10 apenetrating the sleeve 10 in the axial direction and forming acylindrical inner circumferential surface is formed. A small gap isformed between the cylindrical outer circumferential surface of theshaft 4 and the cylindrical inner circumferential surface of the throughhole 10 a, and the shaft 4 is inserted in the sleeve 10. On the side ofone of openings of the hole 10 a (on the bracket 8 side), a first step10 a 1 is formed in correspondence with the thrust plate 6, and a secondstep 10 a 2 having an inner diameter larger than that of the throughhole 10 a, continued from the first step 10 a 1 and, further; having theinner diameter larger than that of the first step 10 a 1 is formed. Agap is formed between a flat surface of the first step 10 a 1 and thetop surface of the thrust plate 6, and a gap is formed between the innercircumferential surface of the first step 10 a 1 and the outercircumferential surface of the thrust plate 6. A thrust bush 16 forclosing an opening of the through hole 10 a is attached to the secondstep 10 a 2. By the thrust bush 16, a gap is formed between the lowersurface of the thrust plate 6 and the side surface of the free end ofthe shaft 4.

[0039] The small gap formed between the inner circumferential surface ofthe through hole 10 a and the outer circumferential surface of the shaft4, the small gap between the flat surface of the first step 10 a 1 andthe upper side surface of the thrust plate 6, the gap between the innercircumferential surface of the first step 10 a 1 and the outercircumferential surface of the thrust plate 6 and, further, the gapbetween the thrust bush 16 and the lower surface of the thrust plate 6are communicated with each other. In the communicated gaps, a lubricantfluid such as oil is held without interruption. An upper end portion (anend portion on the rotor hub 2 side) of the inner circumferentialsurface of the through hole 10 a in the sleeve 10 is tapered so that thedimension in the radial direction of the small gap formed with the outercircumferential surface of the shaft 4 gradually increases to the rotorhub 2 side. The interface of oil is formed between the tapered innercircumferential surface of the through hole 10 a and the outercircumferential surface of the shaft 4 and functions as a taper seal 17.

[0040] (2) Configuration of Bearing

[0041] With reference to FIGS. 1 to 3, the bearing parts will bedescribed.

[0042] As shown in FIG. 2, in the inner circumferential surface of thethrough hole 10 a of the sleeve 10, a series of circumferential dynamicpressure generating grooves is formed by herringbone grooves 18 a eachhaving an almost V shape constructed by connecting a pair of spiralgrooves inclined in opposite directions with respect to the rotatingdirection to the inside in the axial direction of the taper seal 17. Anupper radial hydrodynamic bearing part 18 is constructed between theinner circumferential surface of the through hole 10 a and the outercircumferential surface of the shaft 4.

[0043] The herringbone groove 18 a in the upper radial hydrodynamicbearing part 18 is formed in such a manner that the spiral grooves areasymmetrically with respect to the coupled part, that is, in a shapewhich is unbalanced in the axial direction so that the pumping force ofthe spiral groove provided on the upper side in the axial direction (thetaper seal 17 side) is higher than that of the spiral groove provided onthe lower side in the axial direction (the thrust plate 6 side).

[0044] In the inner circumferential surface of the through hole 10 a ofthe sleeve 10, adjacent to the first step 10 a 1, a series ofcircumferential dynamic pressure generating grooves for inducing a fluiddynamic pressure in the oil at the time of rotation of the rotor 6 isformed by herringbone grooves 20 a each having an almost V shapeconstructed by connecting a pair of spiral grooves inclined in oppositedirections. A lower radial hydrodynamic bearing part 20 is constructedbetween the inner surface of the through hole 10 a and the outercircumferential surface of the shaft 4.

[0045] The herringbone grooves 20 a formed in the lower radialhydrodynamic bearing part 20 are set in such a manner that the spiralgrooves have substantially the same inclination angle with respect tothe rotational axis, groove depth, total length, and width dimension,that is, the spiral grooves are line symmetrical with respect to thecoupled part so that the spiral grooves generate substantially the samepumping forces.

[0046] A series of dynamic pressure generating grooves as spiral grooves22 a are formed concentrically with the thrust plate 6 in the flatsurface of the first step 10 a 1 formed in the sleeve 10, and an upperthrust hydrodynamic bearing part 22 is formed between the flat surfaceof the first step 10 a 1 and the upper surface of the thrust plate 6.The spiral grooves 22 a have, as shown in FIG. 3, a pump-in shape sothat a dynamic pressure to make the oil act to the inside in the radialdirection, that is, onto the shaft 4 side in accordance with therotation of the thrust plate 6 is generated. By the hydrodynamicpressure generated by the spiral grooves 22 a, an axis supporting forcethat the thrust plate 6 acts in the direction of moving apart from thefirst step 10 a 1 is obtained.

[0047] Further, in the inner surface of the thrust bush 16 facing theunder surface of the thrust plate 6 in the axial direction, a series ofdynamic pressure generating grooves as spiral grooves 24 a is formedconcentrically with the thrust plate 6, and a lower thrust hydrodynamicbearing part 24 is formed between the inner surface of the thrust bush16 and the under surface of the thrust plate 6. Like the spiral grooves22 a formed in the upper thrust hydrodynamic bearing part 22, the spiralgrooves 24 a have a pump-in shape so that a dynamic pressure to make theoil act to the inside in the radial direction, that is, to the rotationcenter side of the thrust plate 6 in accordance with the rotation of thethrust plate 6 is generated. By the hydrodynamic pressure generated bythe spiral grooves 24 a, the thrust plate 6 floats from the thrust bush16. Since a concrete shape of the spiral grooves 24 a is similar to thatof the spiral grooves 22 a formed in the upper thrust hydrodynamicbearing part 22, it is not shown but FIG. 3 may be used for reference.

[0048] As described above, by using the spiral grooves 22 a and 24 a asthe dynamic pressure generating grooves formed in the upper and lowerthrust hydrodynamic bearing parts 22 and 24, as compared with the caseof providing herringbone grooves as the dynamic pressure generatinggrooves in the thrust hydrodynamic bearing parts, the efficiency of thebearing is improved.

[0049] Specifically, the herringbone groove has the shape of an almost Vshape constructed by a pair of spiral grooves which are inclined in theopposite directions with respect to the rotation direction. By pumpingoil from both ends of the bearing part to the coupled part of the spiralgrooves at the time of rotation of the rotor, a mountain-shaped pressuredistribution in the axial direction using the coupled part of the spiralgrooves as an apex is obtained.

[0050] In contrast, by the spiral grooves 22 a and 24 a, a pressuredistribution which has an almost trapezoid shape in the radial directionis obtained, in which the pressure is the highest in the center portionof the bearing part, that is, in the outer circumferential portion ofthe shaft 4 in the upper thrust hydrodynamic bearing part, and in therotation axis portion of the shaft 4 in the lower thrust hydrodynamicbearing 24. Therefore, as compared with the case where the herringbonegrooves are provided as the dynamic pressure generating grooves, theeffective area corresponding to load support can be enlarged. In thecase of applying the bearing to a spindle motor of the same load, theoutside diameter of the thrust plate 6 can be reduced and thecircumferential speed is maintained to below. Thus, a loss caused byviscosity resistance of the lubricant fluid can be suppressed.

[0051] In other words, as compared with the case where the herringbonegrooves are applied as the dynamic pressure generating grooves of thethrust hydrodynamic bearing part, while maintaining equivalent loadperformance (load supporting force), a loss is reduced, and the powerconsumption amount of the spindle motor can be suppressed.

[0052] (3) Method of Joining Shaft and Thrust Plate and Configuration ofApparatus used for Joining

[0053] The shaft 4 and the thrust plate 6 are joined to each other by aresistance welding method using a joining apparatus shown in FIG. 4. Theresistance welding method is a method of passing a current to a portionto be welded and applying a pressure while heating the portion by theJoule's heat.

[0054] The joining apparatus has, as shown in FIG. 4, a pair of upperand lower electrodes 102 and 104 connected to an external DC powersource and a current passage control means (not shown) The lowerelectrode 104 is fit in an enclosure 106 having a hollow cylindricalshape. In the enclosure 106, a jig 108 for adjusting the axes of theshaft 4 and the thrust plate 6 is also inserted. The axis adjusting jig108 is constructed by a shaft holding part 108 a in which an axisadjusting hole 108 a 1 penetrating in the axial direction and forpositioning the axis of the shaft 4 in a predetermined position isformed, and an annular thrust plate holding part 108 b which ispositioned in an end on the lower side in the axial direction of theaxis adjusting hole 108 a 1 and is coaxial with the jig adjusting hole108 a 1 and which adjusts the axis of the thrust plate 6 by its innercircumferential surface 108 b 1.

[0055] The thrust plate holding part 108 b has an annular wall whichprojects in the axial direction from the outer side of the innercircumferential surface 108 b 1 by which the thrust plate 6 is held. Theannular wall is attached by means such as bonding to a step provided inthe outer circumferential surface of an lower end in the axial directionof the shaft holding part 108 a (an end portion on the lower electrode104 side). Although the enclosure 106 and the shaft holding part 108 aare made of an insulating ceramic material such as alumina, they may beformed by performing a process such as insulating coating on the surfaceof a conductive material. The thrust plate holding part 108 b is formedof a relatively hard resin such as polyacetal resin. Further, theelectrodes 102 and 104 are coated with an insulating material except forelectrode surfaces 102 a and 104 a.

[0056]FIG. 5 is an enlarged view of a portion around joint surfaces ofthe shaft 4 and the thrust plate 6. An end surface 4 a of the shaft 4and a top surface 6 a of the thrust plate 6 will be called jointsurfaces 4 a and 6 a, respectively, in the following description. Theshaft 4 is made of stainless steel having conductivity such as SUS420Fin consideration of characteristics of hardness, wear resistance and thelike. In the joint surface 4 a, as shown in FIG. 5, a circular recess 40as a melted matter receiving recess and a circumferential projection 42in a position apart from an outer circumferential wall 40 a of therecess 40 are formed so as to surround the axis. The projection 42 isalmost concentric with the recess 40 and has a sectional shape of analmost triangle or trapezoid. The height of the projection 42 is set sothat the projection projects from the joint surface 4 a in the axialdirection by about 0.1 mm to 0.2 mm. The thrust plate 6 is formed of thesame material as that of the shaft 4 in consideration of conductionresistance and the like.

[0057] The shaft 4 and the thrust plate 6 are joined to each other bythe following method by resistance welding. First, the thrust plate 6 ispress fit into the inner circumferential surface 108 b 1 of the thrustplate holding part 108 b, and the shaft 4 is inserted into the axisadjusting hole 108 a 1 in the shaft holding part 108 a until theprojection 42 comes into contact with the joint surface 6 a of thethrust plate 6. Subsequently, the axis adjusting jig 108 is lowered inthe axial direction so as to be fit in the enclosure 106, and the thrustplate 6 is allowed to come into contact with the electrode surface 104 aof the lower electrode 104.

[0058] By setting the inside diameter of the axis adjusting hole 108 a 1to be larger than the outside diameter of the shaft 4 by about 0.002 mmto 0.005 mm and setting the inside diameter of the inner circumferentialsurface 108 b 1 of the thrust plate holding part 108 b to be larger thanthe outside diameter of the thrust plate 6 by about 0.005 mm to 0.01 mm,the axis adjusting jig 108 can be easily attached/detached and thecoaxiality between the axis of the shaft 4 and the center of the thrustplate 6 can be adjusted with precision. Further, the thrust plate 6 ispress fit and held in the inner circumferential surface of the thrustplate holding part 108 b made of resin, variations in the dimensionalprecision caused by process tolerance or the like of the thrust plate 6are absorbed by the thrust plate holding part 108 b, and the centerposition of the thrust plate 6 can be positioned with high precision.

[0059] In a state where the axis of the shaft 4 and the center of thethrust plate 6 are adjusted by the axis adjusting jig 108, the electrodesurface 102 a of the upper electrode 102 is lowered by an energizingmechanism (not shown) until it comes into contact with the upper endsurface of the shaft 4. In a state where the joint surfaces 4 a and 6 aof the shaft 4 and the thrust plate 6 are in contact with each other,while applying a pressure of about 50 kgf to the joint surfaces 4 a and6 a by the energizing mechanism, a direct voltage of about 10 volts (V)is applied across the electrodes 102 and 104 for about 0.003 second. Theeffective current in this case is about 3,000 ampere (A).

[0060] By the operation, the tip of the circumferential projection 42provided for the shaft 4 and the portion in contact with the projection42 in the joint surface 6 a of the thrust plate 6 are melted. When theenergizing is finished, a melted matter is solidified soon, therebycompleting joint between the shaft 4 and the thrust plate 6. At the timeof the joint, both of the members are attracted to each other by metalcoupling of the melted matters and the melted matters flow in the recess40 in the shaft 4 as the joint surfaces 4 a and 6 a approach each other.According to the joining method, a relatively low voltage is appliedonly for short time. Consequently, a generation amount itself of themelted matters is small, and the melted matters do diffuse to the outerside of the outer circumferential wall 40 a of the recess 40. Therefore,the region outside of the recess 40 in the joint surface 4 a of theshaft 4 and the joint surface 6 a of the thrust plate 6 are in directcontact with each other without any intervention of the melted matter,and the perpendicularity of the thrust plate 6 with respect to the axisof the shaft 4 can be assured with high precision. That is, the shaft 4and the thrust plate 6 can be joined to each other while maintainingboth coaxiality and perpendicularity between the shaft 4 and the thrustplate. In addition, the shaft 4 and the thrust plate 6 can be joined toeach other only by applying a relatively low voltage only for shorttime, so that the contact portions between the shaft 4 and the thrustplate 6 and the upper and lower electrodes 102 and 104 can be preventedfrom being melted and occurrence of so-called melt dusts can beprevented.

[0061] By forming the projection 42 circumferentially, the contactbetween the joint surfaces 4 a and 6 a of the shaft 4 and the thrustplate 6 becomes line-contact. Even when the pressure is applied and theresistance welding is performed, a force generated by the pressure andenergization is distributed in the circumferential direction.Consequently, even when the thickness (dimension in the axial direction)of the thrust plate 6 is reduced, occurrence of deflection can besuppressed. Therefore, without sacrificing the support length of theshaft in the radial bearing part, the invention can contribute tofurther reduction in the size and thickness of the spindle motor.

[0062] Moreover, the joint area between the shaft 4 and the thrust plate6 increases by making the projection 42 in a circumferential shape, sothat variations such as insufficient welding strength can be prevented,and stable quality can be assured.

[0063] In addition, also in a spindle motor requested to be small andthin, the thrust plate 6 which is a member separate from the shaft 4 canbe used. Since a mark is not formed by a tool in the root portion of thethrust plate unlike the case where the shaft and the thrust plate areformed integrally, the diameter of each of the shaft 4 and the thrustplate 6 can be reduced. Therefore, viscosity resistance of oil issuppressed and the efficiency can be increased, so that the powerconsumption amount can be reduced.

[0064] (5) Modification of Method of Joining Shaft and Thrust Plate

[0065] In place of the above-described joining method, as shown in FIG.6, it is also possible to provide a circumferential projection 42′ and acircular recess 44 having a diameter smaller than the projection 42′ ina joint surface 4 a′ of the shaft 4 and provide an annular groove 46having a diameter smaller than the outside diameter of the shaft 4 andlarger than the diameter of the projection 42′ in a joint surface 6 a′of the thrust plate 6. In this case, by setting the dimension in theaxial direction of the projection 42′ to about 0.1 mm to 0.2 mm, theshaft 4 and the thrust plate 6 can be joined to each other by resistancewelding by a process similar to the joining method. A matter melted fromthe portion in which the projection 42′ and the joint surface 6 a′ ofthe thrust plate 6 are in contact with each other is housed in thecircular recess 44 and the annular groove 46, so that it is not diffusedto the outside of the annular groove 46. Therefore, the contact betweenthe joint surface 4 a of the shaft 4 in the region on the outside of theannular groove 46 and the joint surface 6 a′ of the thrust plate 6 isnot disturbed by the melted matter, so that effects similar to those inthe embodiment shown in FIG. 5 can be also obtained.

[0066] (6) Another Modification of Method of Joining Shaft and ThrustPlate

[0067]FIGS. 7A and 7B show another modification of the joining method.As shown in FIG. 7A, a circular recess 47 having a diameter smaller thanthe outside diameter of the shaft 4 is provided in a joint surface 6 a″of the thrust plate 6, a circular projection 48 having a diametersmaller than the outside diameter of the circular recess 47 is providedin a joint surface 4 a″ of the shaft 4 and, further, a circumferentialprojection 49 having a diameter smaller than the outside diameter of theprojection 48 is formed in a position apart from an outercircumferential wall 47 a of the recess 47 is formed in the circularrecess 47 in the thrust plate 6. The height of the circular projection48 from the joint surface 4 a″ is set to be slightly smaller than thedepth of the circular recess 47 (for example, smaller by 0.1 mm).

[0068] Also in the case of the modification, the shaft 4 and the thrustplate 6 can be joined to each other by resistance welding by a processsimilar to the joining method. A melted matter from the contact portionbetween the projection 49 of the thrust plate 6 and a surface 48 a ofthe projection 48 of the shaft 4 is housed in the circular recess 47 andis not diffused to the outside of the circular recess 47. Therefore, thecontact between the joint surface 4 a″ of the shaft 4 in the regionoutside of the circular recess 47 and the joint surface 6 a″ of thethrust plate 6 is not disturbed. In this case as well, effects similarto those of the embodiment shown in FIG. 5 can be obtained.Particularly, in the modification, the shaft 4 and the thrust plate 6are welded in such a manner that the projection 48 of the shaft 4 entersthe recess 47 in the thrust plate 6 as shown in FIG. 7B, so that theycan be joined to each other more securely while maintaining both ofcoaxiality and perpendicularity between the shaft 4 and the thrust plate6 with high precision.

[0069] In the modification shown in FIG. 7, the circular recess 47 andthe circumferential projection 49 in the thrust plate 6 can be formed bya press work on the thrust plate 6. In this case, the circular recess 47and the projection 49 can be press molded simultaneously. After thepress work on the thrust plate 6, by grinding or polishing both surfacesof the thrust plate 6, even if the tip of the projection 49 is projectedfrom the joint surface 6 a″, the tip can be removed and the precision ofthe height of the projection can be assured. In the case of obtainingthe thrust plate 6 by press punching a plate member, the circular recess47 and the projection 49 can be also formed simultaneously at the timeof the press work. Moreover, on the shaft 4 side, it is unnecessary toform a recess and a circumferential projection in an end surface of theshaft 4 but it is sufficient to form the circular projection 48concentrically. Thus, the circular projection 48 can be obtained by asimple cutting work, and the merit from the viewpoint of cost is verylarge.

[0070] (7) Configuration of Disk Driving Apparatus

[0071]FIG. 8 is a schematic diagram showing the internal configurationof a general disk driving apparatus 50. In a housing 51, a clean spacein which dusts and the like hardly exist is formed. In the space, aspindle motor 52 to which a disc-shaped disk plate 53 for storinginformation is attached is mounted. In addition, in the housing 51, ahead 56 for reading/writing information from/to the disk plate 53, anarm 55 for supporting the head 56, and an actuator 54 for moving thehead 56 and the arm 55 to a required position over the disk plate 53 aredisposed.

[0072] As the spindle motor 52 of the disk driving apparatus 50, thespindle motor shown in FIG. 1 is used, thereby enabling the thicknessand cost of the disk driving apparatus 50 to be reduced while obtainingdesired rotation precision.

[0073] Although only some exemplary embodiments of this invention havebeen described in detail above, those skilled in the art will readilyappreciated that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

[0074] For example, in the embodiment, the case of forming the shaft 4and the thrust plate 6 by using the same material has been described. Ifthere is no inconvenience for resistance welding, the shaft 4 and thethrust plate 6 can be made of different materials.

[0075] Although the spindle motor of the so-called shaft rotating typein which the shaft 4 and the thrust plate 6 rotate together with therotor hub 2 has been described in the foregoing embodiment, obviously,the invention can be also applied to a spindle motor of a so-calledshaft fixed type in which the shaft and the thrust plate construct apart of a stationary member and to a hydrodynamic bearing used for thespindle motor.

[0076] Further, the material of the sleeve 10 can be properly selectedfrom a solid metal material such as an aluminum material, a coppermaterial, or a stainless steel, a sintered material obtained bysintering copper powders, iron powders, or the like, and the like.

[0077] In addition, the bracket 8 is fixed to the housing (indicated asthe housing 51 in FIG. 7) of the disk driving apparatus by means such asa screw. By integrating the housing and the bracket, the housing can bealso used as the bracket 8.

[0078] Although the foregoing embodiment relates to the case of usingthe thrust plate 6 (as a component of the thrust hydrodynamic bearing)as a disc member, the invention is not limited to the case. Obviously,the invention can be similarly applied to the case of joining a discmember which does not have the thrust hydrodynamic bearing function buthas only a coming-off preventing function to an end surface of theshaft.

What is claimed is:
 1. A hydrodynamic bearing comprising: a shaft memberincluding a shaft having a cylindrical outer circumferential surface andan end surface which is orthogonal to the cylindrical outercircumferential surface, and a disc member having a diameter larger thanthat of said shaft and having a flat surface facing the end surface ofthe shaft, the flat surface being joined and fixed to said end surfaceof the shaft; a bearing member having a cylindrical innercircumferential surface facing the cylindrical outer circumferentialsurface of said shaft and capable of rotating relative to said shaftmember; and a radial hydrodynamic bearing part formed between thecylindrical outer circumferential surface of said shaft and thecylindrical inner circumferential surface of the bearing member, whereinin joint surfaces of said shaft and said disc member, a circumferentialprojection having a diameter smaller than an outside diameter of saidshaft and projecting in the axial direction, and a recess at least ofwhich outer periphery has a diameter smaller than the outside diameterof said shaft and larger than the diameter of the projection and has acircular shape are provided, said projection is melted when apredetermined voltage is applied across said shaft and said disc memberin a state where the joint surface of said shaft and the joint surfaceof said disc member are in contact with each other, the melted matter ishoused in said recess, an end surface of said shaft and a flat surfaceof said disc member are in contact with each other in a portion outsideof said recess, and said shaft and said disc member are integrated bywelding.
 2. The hydrodynamic bearing according to claim 1, wherein saidrecess is a circular recess provided in one of the joint surfaces whichare joined to each other, of said shaft and said disc member, and saidprojection from said recess is projected from the joint surface of saidshaft.
 3. The hydrodynamic bearing according to claim 2, wherein saidrecess and said projection are provided in an end surface of said shaft.4. The hydrodynamic bearing according to claim 1, wherein saidprojection is provided for one of the joint surface of said shaft andthe joint surface of said disc member, which are joined to each otherand, said recess takes the form of an annular groove provided for theother one of the joint surface of said shaft and the joint surface ofsaid disc member.
 5. The hydrodynamic bearing according to claim 4,wherein for one of the joint surfaces of said shaft and said discmember, which are joined to each other, a recess which is recessed fromthe joint surface is provided on the inner circumferential side of saidprojection.
 6. The hydrodynamic bearing according to claim 5, whereinsaid projection is provided for an end surface of said shaft, and saidannular groove is provided in a flat surface of said disc member.
 7. Thehydrodynamic bearing according to claim 1, wherein said projection andsaid recess are provided for one of joint surfaces of said shaft andsaid disc member, which are joined to each other and, for the other oneof the joint surfaces of said shaft and said disc member, which arejoined to each other, a circular projection having an outside diametersmaller than the outer periphery of said recess and larger than saidprojection, and projecting in the axial direction from the joint surfaceis provided.
 8. The hydrodynamic bearing according to claim 7, whereinsaid recess is a circular recess, and said projection is positioned inthe recess.
 9. The hydrodynamic bearing according to claim 7, whereinsaid recess and said projection are provided for a flat surface of saiddisc member, and said projection is provided for an end surface of saidshaft.
 10. The hydrodynamic bearing according to claim 9, wherein saidrecess and said projection are formed by performing a press work on saiddisc member.
 11. The hydrodynamic bearing according to claim 1, whereinsaid bearing member has a bearing surface facing one or both of surfacesof said disc member, and a thrust hydrodynamic bearing part is formedbetween said disc member and said bearing member.
 12. A method ofmanufacturing a hydrodynamic bearing comprising: a shaft memberincluding a shaft having a cylindrical outer circumferential surface andan end surface which is orthogonal to the cylindrical outercircumferential surface, and a disc member having a diameter larger thanthat of said shaft and having a flat surface facing the end surface ofthe shaft, the flat surface being joined and fixed to said end surfaceof the shaft; a bearing member having a cylindrical innercircumferential surface facing the cylindrical outer circumferentialsurface of said shaft and capable of rotating relative to said shaftmember; and a radial hydrodynamic bearing part formed between thecylindrical outer circumferential surface of said shaft and thecylindrical inner circumferential surface of the bearing member,comprising the steps of: providing, in joint surfaces which are joinedto each other of said shaft and said disc member, a circumferentialprojection having a diameter smaller than an outside diameter of saidshaft and projecting in the axial direction, and a recess at least ofwhich outer periphery has a diameter smaller than the outside diameterof said shaft and larger than the diameter of the projection and has acircular shape; applying a predetermined voltage across said shaft andsaid disc member in a state where a pressure is applied in the axialdirection to said shaft and said disc member so that the joint surfaceof said shaft and the joint surface of said disc member are in contactwith each other; melting said projection until the end surface of saidshaft and the flat surface of said disc member come into contact witheach other in a region outside of said recess and allowing a meltedmatter of the projection to enter said recess; and thereby fixing saidshaft and said disc member by welding.
 13. A method of manufacturing ahydrodynamic bearing comprising: a shaft member including a shaft havinga cylindrical outer circumferential surface and an end surface which isorthogonal to the cylindrical outer circumferential surface, and a discmember having a diameter larger than that of said shaft and having aflat surface facing the end surface of the shaft, the flat surface beingjoined and fixed to said end surface of the shaft; a bearing memberhaving a cylindrical inner circumferential surface facing thecylindrical outer circumferential surface of said shaft and capable ofrotating relative to said shaft member; and a radial hydrodynamicbearing part formed between the cylindrical outer circumferentialsurface of said shaft and the cylindrical inner circumferential surfaceof the bearing member, comprising the steps of: providing a circularrecess having a diameter smaller than the outside diameter of said shaftand recessed in the axial direction, in one of the joint surfaces whichare joined to each other, of said shaft and said disc member, and acircumferential projection which is projected in the axial directionfrom said one of the joint surfaces in the recess; applying apredetermined voltage across said shaft and said disc member in a statewhere said shaft and said disc member are pressed against each other ina direction orthogonal to the axial direction so that the joint surfaceof said shaft and the joint surface of said disc member are in contactwith each other; melting said projection until the end surface of saidshaft and the flat surface of said disc member come into contact witheach other in a region outside of said recess and allowing a meltedmatter of the projection to enter said recess; and thereby fixing saidshaft and said disc member by welding.
 14. A method of manufacturing ahydrodynamic bearing comprising: a shaft member including a shaft havinga cylindrical outer circumferential surface and an end surface which isorthogonal to the cylindrical outer circumferential surface, and a discmember having a diameter larger than that of said shaft and having aflat surface facing the end surface of the shaft, the flat surface beingjoined and fixed to said end surface of the shaft; a bearing memberhaving a cylindrical inner circumferential surface facing thecylindrical outer circumferential surface of said shaft and capable ofrotating relative to said shaft member; and a radial hydrodynamicbearing part formed between the cylindrical outer circumferentialsurface of said shaft and the cylindrical inner circumferential surfaceof the bearing member, comprising the steps of: providing an annulargroove having a diameter smaller than the outside diameter of said shaftand recessed in the axial direction, in one of the joint surfaces whichare joined to each other, of said shaft and said disc member, and acircumferential projection which has a diameter smaller than that ofsaid annular groove and is projected in the axial direction from theother one of the joint surfaces and a recess having a diameter smallerthan the projection in the other one of the joint surfaces which arejoined to each other of said shaft and said disc member; applying apredetermined voltage across said shaft and said disc member in a statewhere said shaft and said disc member are pressed against each other ina direction orthogonal to the axial direction so that the joint surfaceof said shaft and the joint surface of said disc member are in contactwith each other; melting said projection until the end surface of saidshaft and the flat surface of said disc member come into contact witheach other in a region outside of said annular groove and allowing amelted matter of the projection to enter said annular groove and/or saidrecess; and thereby fixing said shaft and said disc member by welding.15. A method of manufacturing a hydrodynamic bearing comprising: a shaftmember including a shaft having a cylindrical outer circumferentialsurface and an end surface which is orthogonal to the cylindrical outercircumferential surface, and a disc member having a diameter larger thanthat of said shaft and having a flat surface facing the end surface ofthe shaft, the flat surface being joined and fixed to said end surfaceof the shaft; a bearing member having a cylindrical innercircumferential surface facing the cylindrical outer circumferentialsurface of said shaft and capable of rotating relative to said shaftmember; and a radial hydrodynamic bearing part formed between thecylindrical outer circumferential surface of said shaft and thecylindrical inner circumferential surface of the bearing member,comprising the steps of: providing a circular recess having a diametersmaller than the outside diameter of said shaft and recessed in theaxial direction and a circumferential projection positioned in therecess and projected in the axial direction, in one of the jointsurfaces which are joined to each other, of said shaft and said discmember; providing, in the other one of the joint surfaces which arejoined to each other, of said shaft and said disc member, a circularprojected part which has a diameter smaller than that of said recess andlarger than that of said projection and is projected in the axialdirection from the other one of the joint surfaces only by a dimensionsmaller than a depth of said recess; applying a predetermined voltageacross said shaft and said disc member in a state where said shaft andsaid disc member are pressed against each other in a directionorthogonal to the axial direction so that the joint surface of saidshaft and the joint surface of said disc member are in contact with eachother; melting said projection until said projected part enters saidrecess and the end surface of said shaft and the flat surface of saiddisc member come into contact with each other in a region outside ofsaid recess and allowing a melted matter of the projection to enter saidrecess; and thereby fixing said shaft and said disc member by welding.16. An apparatus for manufacturing a shaft member for a hydrodynamicbearing, for joining a disc member to an end surface of a shaft so as tobe substantially orthogonal to the axis of the shaft by resistancewelding, comprising: a pair of electrodes disposed so as to face eachother in the axial direction in order to press said shaft and said discmember against each other in a direction orthogonal to the axialdirection and to apply a predetermined voltage to said shaft and saiddisc member; and an axis adjusting jig for making the axis of said shaftand a center position of said disc member coincide with each other,wherein said axis adjusting jig comprises: a cylindrical shaft holdingpart in which said shaft is inserted, thereby positioning the axis ofsaid shaft in a predetermined position and holding said shaft; and anannular-shaped disc member holding part in which said disc member ispress fit, thereby positioning the center position of said disc memberin a predetermined position and holding said disc member, and the discmember holding part is made of a resin.
 17. A spindle motor comprising:a shaft member including a shaft having a cylindrical outercircumferential surface and a disc member joined to an end surface ofthe shaft; a bearing member having a cylindrical inner circumferentialsurface facing the cylindrical outer circumferential surface of saidshaft and capable of rotating relative to said shaft member; a radialhydrodynamic bearing part formed between the cylindrical outercircumferential surface of said shaft and the cylindrical innercircumferential surface of the bearing member; a rotor coupled to one ofsaid shaft and said bearing member and having a rotor magnet; and astator constructing a stationary member in cooperation of the other oneof said shaft and said bearing member and disposed so as to face saidrotor magnet, wherein in joint surfaces which are joined to each otherof said shaft and said disc member, a circumferential projection havinga diameter smaller than the outside diameter of said shaft andprojecting in the axial direction and a recess of which at least outerperiphery has a diameter smaller than the outside diameter of said shaftand larger than the projection are provided, said projection is meltedwhen a predetermined voltage is applied across said shaft and said discmember in a state where the joint surfaces of said shaft and said discmember are in contact with each other, a melted matter is housed in saidrecess, and an end surface of said shaft and a flat surface of said discmember come into contact with each other in a portion outside of saidrecess, thereby integrating said shaft and said disc member by welding.18. A recording disk apparatus comprising a spindle motor according toclaim 17, wherein a recording disk is mounted on said rotor so as torotate integrally with the rotor, and a recording/reproduction head forreading/writing information from/to the recording disk is provided.